1. Field of the Invention
This invention relates in general to communication devices with antennas, and more particularly, to antennas that can be retracted.
2. Description of the Related Art
In today's marketplace, consumers have access to numerous communication devices such as cellular telephones and personal radios, which use antennas. In order to enable the various operations of such devices, several manufacturers have developed innovative and capable antenna systems. For example, some manufacturers provide communication devices that utilize retractable antennas, while others dispense with external antennas altogether, providing antennas that are fully integrated into the body of the communication device. Still others provide antennas of a fixed length, which may protrude from the communication device.
There are two types of antennas that are generally used in communication devices—one-quarter wavelength antennas and one-half wavelength antennas, depending on desired antenna characteristics. One-half wavelength antennas typically have two elements, each of which is capable of operating in a one-quarter-wavelength configuration. The two one-quarter wavelength elements can be brought into electrical contact with each other, to form a one-half wavelength configuration. The same antenna can also be operated in a one-quarter-wavelength configuration, if the two antenna elements are disconnected electrically.
The problem with the one-half wavelength antennas used in cellular devices is that the efficiency of an antenna in a retracted configuration is significantly lower than that of a dedicated one-quarter-wavelength antenna. As an example, one-half wavelength antennas include two parts—a one-quarter wavelength portion of straight wire and a one-quarter wavelength portion of helical wire. In the retracted configuration, the antenna's main radiator is the helical wire section. In this configuration, the one-quarter-wavelength straight-wire section needs to be electrically disconnected from the one-quarter-wavelength helical wire portion. Antenna designers have attempted to resolve the problem by using electrical grounding techniques and other techniques, such as using matching circuits, to achieve a high degree of disconnection. These techniques fail to completely and properly disconnect the one-quarter-wavelength straight wire portion from the one-quarter-wavelength helical wire portion. Not being able to accomplish a proper electrical disconnection results in a failure to correctly operate the antennas in the one-quarter-wavelength configuration.